A conventionally known vaporized fuel treatment device includes a sealing valve that is driven by a stepper motor and controller that executes a learning of a valve opening start position of the sealing valve based on a change in an internal pressure of a fuel tank when changing an axial distance between a valve element and a valve seat of the sealing valve (as shown in, for example, Patent Literature 1). The controller of the vaporized fuel treatment device moves the valve element by a predetermined stroke at predetermined intervals from a valve closing limit position of the sealing valve and determines whether or not an internal pressure of the fuel tank decreases by a predetermined value or more with respect to a last detected value. Then, the controller judges that the sealing valve starts opening when determining that the internal pressure of the fuel tank decreases by the predetermined value or more with respect to the last detected value and calculates a learning value of the valve opening start position based on a total stroke from the valve closing limit position. Further, the controller determines whether or not an increase amount of the internal pressure of the fuel tank is permissible and interrupts or inhibits the learning of the valve opening start position when determining that the increase amount of the internal pressure of the fuel tank is not permissible during or before the learning.